This invention relates to a fixing device for fixing onto a copying paper a toner image which is transferred from a photosensitive body to the copying paper, more specifically to a fixing device for fixing a toner image onto a copying paper by heating and pressurizing operations.
In a conventional electrostatic copying apparatus, for example, a fixing device for fixing an unfixed toner image is so constructed that resinous powder in the toner is melted by heating and pressurized to be fixed onto a copying paper. In this fixing device, while the copying paper is held under a given pressure between a pair of rollers at least one of which is covered with a heat-resisting elastic material, such as silicone rubber, for a sufficient nip width, the toner image is heated by a heater contained in one of the rollers. If the rollers are left pressed against each other, the elastic material will suffer permanent deformation. As a result, dents will be formed in the surfaces of the rollers to cause partially inadequate pressurization during rotation of the rollers, which will lead to defective fixation or excessive noise.
Whereupon, there was developed a fixing device 10 as shown in FIG. 1. The fixing device 10 has a pair of pressure rollers 14a and 14b facing each other with a conveyor path 12 between them. The one roller 14a can touch and leave the other roller 14b, and contains a heater 16 therein. The roller 14a is connected with a pressure spring 18 for urging the roller 14a toward the roller 14b, and a pressure release mechanism 20 for optionally separating the roller 14a from the roller 14b against the urging force of the pressure spring 18. The fixing device of this type, however, is complicated in construction, and requires a delay mechanism which is intended to release pressure if the power supply is cut while the rollers are rotating.